1. Field of the Invention
The present invention relates to an organic electroluminescent element capable of luminescence by converting electric energy to light.
2. Description of the Related Art
Organic electroluminescent (EL) elements have attracted attention as a promising display device, since they can provide luminescence having high brilliance at a low voltage. One of the important characteristic values of the organic electroluminescent devices is external quantum efficiency. External quantum efficiency is calculated from the following formula:External quantum efficiency φ=(number of photons emitted from device)/(number of electrons injected to device)
As the value increases, the device becomes advantageous in view of power consumption.
Specifically, the external quantum efficiency of the organic electroluminescent device is determined by the following formula.External quantum efficiency φ=(internal quantum efficiency)×(light extraction efficiency)
The threshold value of the internal quantum efficiency of organic EL devices utilizing fluorescent light emitted from an organic compound is 25%, and the light extraction efficiency thereof is about 20%. Therefore, the threshold value of the external quantum efficiency thereof is thought to be about 5%.
An organic electroluminescent device including a triplet luminescent material (phosphorescent material) has been reported as an organic electroluminescent device which has an improved internal quantum efficiency, and in turn an improved external quantum efficiency (for example, see WO 2000/070655). The external quantum efficiency of this device can be much higher than that of a conventional device utilizing emission of fluorescent light (singlet luminescent device), and the maximum value of external quantum efficiency of the device reaches 8% (external quantum efficiency at 100 cd/m2 is 7.5%). However, the durability (excellent durability of luminescence) is not sufficient. An organic electroluminescent device which has both high efficiently and high durability has been demanded.